Firefighting foam concentrates contain mixtures of surfactants that act as foaming agents, together with solvents and other additives that provide the desired mechanical and chemical properties for the foam. The concentrates are mixed with water in situ and foamed by mechanical means, and the resulting foam is projected onto the fire, typically onto the surface of a burning liquid. The concentrates are typically used at a concentration of about 1-6%.
Aqueous film-forming foam (AFFF) concentrates are designed to spread an aqueous film on the surface of hydrocarbon liquids, which increases the rate at which the fire can be extinguished. This spreading property is made possible by the use of perfluoroalkyl surfactants in AFFF, which produce very low surface tension values in solution (15-20 dynes cm−1), thereby permitting the aqueous solution to spread rapidly on the surface of the hydrocarbon liquids.
However, typical AFFF foams are not effective on fires of water-miscible fuels, such as low molecular weight alcohols, ketones, and esters and the like, because the miscibility of the solvent leads to dissolution and destruction of the foam by the fuel. To address this issue, alcohol resistant AFFF (AR-AFFF) concentrates are used, which contain a water-soluble polymer that precipitates on contact with a water-miscible fuel, creating a protective layer between the fuel and the foam. Typical water-soluble polymers used in AR-AFF are polysaccharides, such as xanthan gums. AR-AFFF foams are effective on both hydrocarbon and water-soluble fuels.
Conventional AFFF concentrates contain mixtures of perfluoroalkyl and non-fluorinated surfactants, each of which may be anionic, cationic, nonionic or amphoteric, solvents such as glycols and/or glycol ethers, and minor additives such as chelating agents, pH buffers, corrosion inhibitors and the like. Various AFFF concentrates are described in, for example, U.S. Pat. Nos. 3,047,619; 3,257,407; 3,258,423; 3,562,156; 3,621,059; 3,655,555; 3,661,776; 3,677,347; 3,759,981; 3,772,199; 3,789,265; 3,828,085; 3,839,425; 3,849,315; 3,941,708; 3,957,075; 3,957,657; 3,957,658; 3,963,776; 4,038,198; 4,042,522; 4,049,556; 4,060,132; 4,060,489; 4,069,158; 4,090,976; 4,099,574; 4,149,599; 4,203,850; and 4,209,407. ARAFFF concentrates are described in, for example, U.S. Pat. Nos. 4,060,489; 4,149,599 and 4,387,032.
Foam concentrates often must be stored and used at low temperatures. For example, the concentrate often must be stored outdoors in a cold climate, or it may be stored inside a refrigerated warehouse. Concentrates intended for use under such conditions therefore contain one or more freeze suppressants, that depress the freezing point of the concentrate so that it remains liquid and can be proportioned to make foam. Typically, either propylene glycol or ethylene glycol are used as the freeze suppressant.